In WAN (Wide Area Network) and LAN (Local Area Network), common use of light as a transmission medium has already been started by now. In some cases as in the case of FTTH (fiber to the home), development has reached a stage where an optical signal is transmitted all the way to individual homes.
With optical communications of today, an optical signal can be branched at a ratio of 1 to N (multitude), which is however still an extension of point-to-point communication, and dynamic optical communications capable of switching an optical signal in a state of light as it is has not yet been put to practical use.
There has been proposed an optical packet network system as a technology for implementing futuristic dynamic optical communications. The technology is represented by a network wherein a data signal is converted into optical packet data to be switched in a state of light without conversion into an electrical signal before reaching a terminal.
FIG. 3 is a block diagram showing a conventional example of an optical packet switching network of a ring type.
The example in FIG. 3 shows a state where four nodes 1 to 4 are connected to a ring 5, and an electrical signal inputted to light at the wavelength of a laser diode 13 via an optical modulator 14 is inputted to the node 3.
Further, with this example, there is shown a state where in the case of light noise from the node 3, at light intensity ratio of −15 dB, being merged (ADD) with an optical packet signal A having passed through the node 2, at the node 3, a signal is degraded due to distortion, as in the case of an optical packet signal B outputted from the node 3. Further, a configuration including the laser diode 13 and the optical modulator 14, in a region of the figure, surrounded by a two-dot chain line D, is similarly provided for the other nodes although not shown in figure.
In general, an extinction ratio not lower than 30 dB is regarded as necessary, but it is deemed considerably difficult to implement such an optical modulator as meets such a specification.
With the optical packet network, it is ideal that an optical signal does not exit at all therein at the time when no signal is transmitted. However, in practice, respective nodes or equipment connected thereto make use of an optical modulator when converting an electrical signal into an optical signal. The conventional example of an optical packet ring network system is disclosed in the following Patent Document 1.
[Patent Document 1] JP 2001-36557 A